1. Field of the Invention
The present invention relates to a focus detecting device adapted for use in automatic focusing of a video camera, a television camera or the like.
2. Related Background Art
There is already known a method of automatic focusing, for example in a video camera, of detecting the sharpness of an image on an image sensor from image signals obtained therefrom and moving a photographing lens so as to maximize said sharpness. In such method it has been necessary to vary the focus state of the image on the image sensor by suitable means and to detect the resulting change in sharpness of said image, since the moving direction of the photographing lens cannot be identified from a single image.
For this reason there has been employed a method of axially vibrating a part of lens elements in the photographing lens continuously by a small amount to constantly vary the focus state of the image on the image sensor, or a method of maintaining a fixed focus state after an in-focus state is reached, then inspecting an eventual change in the sharpness of the image caused by the movement of camera or object, and, in case of such change, moving the photographing lens in an arbitrary direction to identify a correct moving direction from a change in sharpness resulting from such lens movement. However, in the former method, deterioration of image quality is inevitable since the image on the image sensor is always slightly out of focus, and detection of change in sharpness becomes difficult if the amount of image blur is reduced in order to avoid deterioration of the image quality. On the other hand, the latter method tends to provide an unstable image since the photographing lens may be driven by an article eventually crossing in front of the object.